During this research in Costa Rica, 11 strains identified as L. braziliensis s.l. have been isolated in cultures from humans. Antigens prepared from a human and an animal species of Leishmania (L. mexicana and L. hertigi) have shown good correlation of immediate and delayed skin reactions in humans. Only 4 of 49 wild animals examined yielded positive cultures for hemoflagellates but none correspond to human Leishmania strains. Among 6,000 sandflies captured, 28 species were identified from the Atlantic side and 20 from the Pacific. L. olmeca, the vector of L. mexicana in other countries, has been found in different areas of Costa Rica. Dissection of 160 females of different species yielded 10 specimens positive for hemoflagellates which were isolated in cultures (6 from L. shannoni and 4 from L. ylephiletor). None of these has shown the characteristics of L. braziliensis on hamster inoculation or tissue culture passage. The species of sandflies captured by different traps or methods were compared; nine have been identified biting humans. Monkeys and hamsters were used as models for different immunological experiments. Monkeys were more susceptible to L. braziliensis than to L. mexicana and one cured Cebus had complete immunity to challenge with an heterologous strain of the former species. Both Freund's complete adjuvant and a lysate of culture parasites modified the course of infections produced by strains of both species in hamsters by arresting partially or totally the development of the lesions. Splenectomy or simultaneous injection of both species in the nose and paw did not modify the course of the infections. BIBLIOGRAPHIC REFERENCES: Zekedib, R., Ponce, C., and Elisa de Ponce. 1977. Finding of Leishmania hertigi in the Costa Rican porcupine. Journal of Parasitology, in press. Zeledon, R., McPherson, B., and Ponce, C. 1977. Isolation of Leishmania braziliensis from a wild rodent in Costa Rica. American Journal Tropical Medicine and Hygiene, in press.